In recent years, gas tanks (gas cylinders) that store hydrogen or natural gas serving as fuel for electric power generation have been used in automobiles, houses, transport machinery, and the like.
For instance, polymer electrolyte fuel cells have been gaining attention as a power source for automobiles. When such fuel cells are used for electric power generation, an electrochemical reaction is induced by supplying a gas fuel (e.g., hydrogen gas) to a gas diffusion electrode layer provided on one side of each fuel cell and supplying an oxidant gas (e.g., air containing oxygen) to a gas diffusion electrode layer provided on the other side. Upon such electric power generation, nontoxic water is exclusively produced. Thus, the above fuel cells have been gaining attention from viewpoints of environmental influences and use efficiency.
In order to continuously supply a gas fuel such as hydrogen gas to an automobile equipped with the above fuel cells, a gas fuel is stored in an in-vehicle gas tank. Examples of in-vehicle hydrogen gas tanks that have been examined include a gas tank that stores compressed hydrogen and a hydrogen-storing gas tank that stores hydrogen in a state of absorption in metal hydride (MH).
Among them, a CFRP (carbon fiber-reinforced plastic) tank has been examined for use as an in-vehicle gas tank that stores compressed hydrogen. A CFRP tank is structured such that a liner layer (inner shell) that maintains airtight properties of the tank is formed inside a layer (outer shell: fiber-reinforced layer) comprising a carbon fiber-reinforced plastic (CFRP material). Such CFRP tank has strength greater than that of a tank made of a usual type of plastic and is excellent in pressure resistance, and therefore it is preferably used as a gas fuel tank.
As an aside, a high-pressure hydrogen container (compressed hydrogen gas tank: CHG tank) system in a fuel-cell vehicle is loaded with high-pressure hydrogen gas (between 35 MPa and 75 MPa or more). In such case, in terms of the degree of freedom of sealing material design, sealing with the use of elastomer is more desirable than sealing with the use of metal material. In addition, the development of material that has durability against filling and discharge of a high-pressure hydrogen gas at high frequency is awaited. Hydrogen gas incorporated into an elastomer at high pressures tends to diffuse outside the elastomer under reduced pressure so that it is necessary for such material to be durable in variable pressure environments. Further, it is necessary for such material to be durable in variable temperature environments (approximately between a low temperature of −70° C. and a high temperature of 80° C.).
There are a variety of known sealing materials that are generally used. For instance, JP Patent Publication (Kokai) No. 10-182882 A (1998) discloses a rubber composition comprising a specific hydrogenated nitrile rubber (a) to which a specific carbon black (b) has been added, such carbon black having specific surface area, compressed DBP oil absorption amount, tint strength, ratio of specific surface area for nitrogen adsorption to iodine adsorption amount, and electron-microscopically-observed average particle size. This is because, when conventional materials obtained by adding silicon dioxide to hydrogenated nitrile rubber are used for molding of sealing members for car air-conditioner compressors, the sealing members obtained by vulcanization molding of such materials are not satisfactory in terms of fluorohydrocarbon-resistant properties (blister resistance) and wear resistance (necessary for movable sealing members) under high temperature conditions. The reference also describes that a product obtained by vulcanization molding of such rubber composition, which is used for sealing members and the like for car air-conditioner compressors, is excellent in blister resistance, wear resistance, and the like.
In addition, in Plast Rubber Compos Process Appl (JIN: D0988B; ISSN: 0959-8111) VOL. 22, No. 3, an elastomer was theoretically analyzed in terms of liquid absorption, high-pressure permeation, and rapid disintegration (explosive disintegration), with the title of “Durability of TFE/P and other fluorinated elastomers when used in stringent high-pressure environments for sealing purposes.” The obtained results were further confirmed by experimentation. The reference also describes that sealing materials tend to deteriorate due to physical influences rather than chemical reactions. In addition, the reference introduces, as a fluorinated elastomer, an elastomer (explosion-proof elastomer) that is excellent in terms of durability against rapid disintegration (explosive disintegration).
However, an explosion-proof elastomer is significantly inferior in “sag resistance,” which is important for sealing duration performance, and in “low-temperature properties (retraction properties),” which are important in an environment in which a high-pressure hydrogen tank for fuel cells is used. These issues have been problematic.
It is considered that the above problems have occurred for following reasons.
(1) The crosslink density of a fluorinated elastomer is excessively increased; that is to say, an elastomer is formed into an ebonite material in a manner such that the elastomer is modified in order to improve explosion-proof properties of an explosion-proof elastomer. This results in loss of retraction properties essentially imparted to an elastomer.(2) The amount of gas absorption in an elastomer is suppressed in order to improve explosion-proof properties. Specifically, the composition of an elastomer is modified such that the polymer fraction is lowered (the polymer fraction is lowered in a mixed composition). Such modification is considered to result in impairment of the elastomer characteristics, leading to deterioration in sag resistance.(3) A fluorinated elastomer is essentially inferior in low-temperature properties. In addition, low-temperature properties deteriorate as a result of the modifications described in (1) and (2) above.